Global communication systems having digital transmission pipes are being subjected to massive amounts of digital information including both voice and data signals. To make more efficient use of these pipes, the original information can be compressed at the sender before being launched into the pipe. The compressed information is then decompressed at the receiver into its original form before it can be used. For instance, speech coders are optimized to compress voice signals by taking advantage of certain characteristics of human speech patterns.
Recently, several popular low bit-rate speech coders/decoders have been developed which can compress a 64 kbit/sec digitized voice signal into 16 kbit/sec or lower, and still provide a relatively clear sounding, decompressed voice signal. However, such low bit-rate speech codecs, and particularly those which compress to 8 kbit/sec or lower, exhibit poor performance with non-voice signals. These non-voice signals include single or multi-tone signals that are generated by pressing the digits on a telephone keypad, such as for accessing voicemail and bank account records. The low bit-rate speech codec distorts these tone signals, such that the use of conventional techniques for detecting tone signals in the reconstructed sequence of decoded data have been unreliable in detecting the proper digit that was pressed.